This would be just like thorns; if a monster attacks you from melee range, they would take the damage listed. If you have more than one item with this property, they will accumulate to provide a set amount of damage whenever you get hit.

If I attack I am an attackER. If I am attacked, I am attacked by an attackER. Yes?
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ThariusMay 16 '12 at 13:40

It's a quote, not my wording. The link provides a discussion that in context explains that it is like Thorns, and many posters specifically mention that it is like Thorns. Also, I don't believe the game ever refers to you as an attacker for personal abilities. Personal abilities - attacks, effects on others (allies and enemies) - attackers.
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EBongoMay 16 '12 at 13:45